1. Field of the Invention
The present invention relates to a semiconductor device having a semiconductor element such as a transistor formed thereon.
2. Description of the Related Art
A semiconductor device formed on a silicon-on-insulator (SOI) substrate is provided with an isolation film (a field oxide film) for electrically isolating (insulating) a semiconductor element (transistor element) formed on a silicon layer from others.
Local oxidation of silicon (abbreviated as LOCOS) is a method to form the field oxide film. The LOCOS (or selective oxidation) is a method of forming an oxide film through a thermal oxidation process with respect to a silicon layer with a nitride film as a mask, thereby forming a field oxide film.
Use of an isolation method based on the LOCOS (LOCOS isolation) generates a region, commonly called a bird's beak, where the field oxide film grows thin, at the end of a field oxide film, more specifically, at the boundary with a region where a semiconductor element is formed (an active region).
At the bird's beak, thinning of the active layer in the SOI substrate causes lowering of a threshold voltage in a gate voltage of the transistor element.
Such lowering of the threshold voltage causes the leakage (leak current) at the bird's beak by the operation (turn-on) of the parasitic transistor before the start of the operation of a transistor at a designed value, that is, before sufficient rise in the gate voltage.
A conventional art for suppressing such a leak current at a bird's beak is disclosed in Japanese Patent Application JP 2003-124303A.
The disclosure proposes a technique for suppressing leak current by implanting an impurity between a bird's beak formed at an end of a field oxide film and an insulating layer (BOX oxide film layer) of an SOI substrate, that is, in a region where a channel (path of electrons) is formed at a voltage lower than a predetermined gate voltage.
By the disclosure, a precise condition setting for implanting an impurity (field doping) is required while considering many factors such as a concentration of the impurity to be implanted and a processing temperature for the impurity not to be implanted in a standard channel region. Accordingly, implanting of an impurity for suppressing leak current has been difficult.